1. Field of the Invention
This invention relates to improved catalysts for hydrodemetalizing and hydrodesulfurizing metal and sulfur-contaminated hydrocarbon oils, especially petroleum fractions such as residual oils. The invention further relates to the preparation and use of such catalysts and the alumina supports thereof.
2. Description of the Prior Art
Hydrocarbon oils may be obtained from various sources such as petroleum, tar sands, coal and oil-bearing shale. Since petroleum is presently the principal source of hydrocarbon oils, the discussion which follows will refer to this source with the understanding that the present invention is applicable to metal and sulfur-contaminated oils regardless of source.
Crude petroleum is separated by distillation into fractions of increasing molecular weight. In general, the demand for the lighter fractions such as gasoline and kerosine exceeds the amount available from distillation alone. Petroleum refiners therefore resort to cracking and hydrocracking the heavier fractions, and to other processes, to convert these to more desirable products. These conversions, particularly cracking and hydrocracking, are generally employed with heavy distillate fractions, however, for reasons hereinbelow described.
Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by relatively high metals and sulfur content. This comes about because practically all of the metals present in the original crude remain in the residual fraction, and a disproportionate amount of sulfur in the original crude oil also remains in that fraction. Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper also sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. The high metals content of the residual fractions generally preclude their effective use as charge stocks for subsequent catalytic processing such as catalytic cracking and hydrocracking. This is so because the metal contaminants deposit on the special catalysts for these processes and cause the premature aging of the catalyst and/or formation of inordinate amounts of coke, dry gas and hydrogen.
It is current practice to upgrade certain residual fractions by a pyrolitic operation known as coking. In this operation the residuum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree. to 1100.degree. F. temperature and a pressure of one to ten atmospheres. The economic value of the coke by-product is determined by its quality, especially its sulfur and metals content. Excessively high levels of these contaminants makes the coke useful only as low-valued fuel. In contrast, cokes of low metals content, for example up to about 100 p.p.m. (parts-per-million by weight) of nickel and vanadium, and containing less than about 2 weight percent sulfur may be used in high valued metallurgical, electrical, and mechanical applications.
Certain residual fractions are currently subjected to visbreaking, which is a heat treatment of milder conditions than used in coking, in order to reduce their viscosity and to make them more suitable as fuels. Again, excessive sulfur content sometimes limits the value of the product.
Residual fractions are sometimes used directly as fuels. For this use, a high sulfur content in many cases is unacceptable for ecological reasons.
At present, catalytic cracking is generally done utilizing hydrocarbon chargestocks lighter than residual fractions which generally have an API gravity less than 20. Typical cracking chargestocks are coker and/or crude unit gas oils, vacuum tower overhead, etc., the feedstock having an API gravity from about 15 to about 45. Since these cracking chargestocks are distillates, they do not contain significant proportions of the large molecules in which the metals are concentrated. Such cracking is commonly carried out in a reactor operated, in the absence of added hydrogen, at a temperature of about 800.degree. to 1500.degree. F., a pressure of about 1 to 5 atmospheres, and a space velocity of about 1 to 1000 WHSV.
Typical hydrocracking reactor conditions consists of a temperature of 400.degree. to 1000.degree. F. and a pressure of 100 to 3500 p.s.i.g.
The amount of metals present in a given hydrocarbon stream is often expressed as a chargestock's "metals factor". This factor is equal to the sum of the metals concentrations, in parts per million, of iron and vanadium plus ten times the concentration of nickel and copper in parts per million, and is expressed in equation form as follows: EQU F.sub.m =Fe+V+10(Ni+Cu)
Conventionally, a chargestock having a metals factor of 2.5 or less is considered particularly suitable for catalytic cracking. Nonetheless, streams with a metals factor of 2.5 to 25, or even 2.5 to 50, may be used to blend with or as all of the feedstock to a catalytic cracker, since chargestocks with metals factors greater than 2.5 in some circumstances may be used to advantage, for instance with the newer fluid cracking techniques.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals factor. As an example, a typical Kuwait crude, considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80% of the metals and preferably at least 90% needs to be removed to produce fractions (having a metals factor of about 2.5 to 50) suitable for catalytic cracking or hydrocracking chargestocks.
Catalysts and processes that utilize such catalysts have been proposed to hydrodemetalize and hydrodesulfurize metal and sulfur-contaminated hydrocarbon oils such as residual petroleum fractions. Such are described for example, in U.S. Pat. Nos. 3,891,541; 3,931,052; 4,016,067 and 4,054,508. Other catalysts and processes directed primarily to removal of sulfur and distillate or residual oils to provide low-sulfur fuels have been proposed.
In the latter category, attention is called to U.S. Pat. No. 3,975,303 issued Aug. 17, 1976 to Eyles which describes a hydrodesulfurization catalyst comprising 1-10% of an iron group metal, 5-25% of a Group VIB metal, and 0.1-10% of a rare earth on a refractory support. The rare earth improves the desulfurization ability of the catalyst and suppresses demetalation.
As a matter of convenience, the catalyst of the present invention will be referred to simply as a hydrodemetalation catalyst since it very effectively reduced the metal content of a treated oil. It is to be understood, of course, that it is effective also for removing sulfur. Catalysts made for hydrodesulfurizing oils also may remove some metal, but this removal is generally regarded as undesirable since it tends to prematurely age the catalyst for its intended use.